Field of Invention
The present invention relates to a multi-phase motor control method, especially a method for controlling a phase switching frequency of a multi-phase motor according to a phase difference, and a multi-phase motor using the method.
Description of Related Art
FIG. 1A shows a control circuit of a y-connection 3-phase sensor-less permanent magnet synchronous motor, and FIGS. 1B-1C show its operation process. The horizontal coordinates 0° to 360° indicate an electrical cycle of the motor stator, and the vertical coordinates Vu, Vv, and Vw respectively indicate the voltages corresponding to nodes Vu, Vv, and Vw of FIG. 1A; that is, Vu, Vv, and Vw are the driving voltage signals sensed at the three nodes of the 3-phase motor. The curve Vc in a zigzag shape shown in FIGS. 1B-1C indicates the voltage at the neutral node Vc of FIG. 1A. (Vu, Vv, Vw and Vc are used to refer to the nodes or the voltages at the nodes, according to the context.)
Taking the six-step square-wave diving method as an example, the phase switching timing of the 3-phase sensor-less permanent magnet synchronous motor is determined according to zero-crossing points (Pz, FIGS. 1B-1C) between a back electromotive force (BEMF) and floating phase voltages at nodes Vu, Vv, and Vw. When the zero-crossing points are sensed, the phase switching of the 3-phase sensor-less permanent magnet synchronous motor is ready to perform. For example, referring to FIGS. 1A-1C, in the stage from 0° through 60°, the node Vu charges the node Vv through the neutral node Vc, and the node Vw is floating. The voltage Vw is compared with the voltage Vc, and when the zero-crossing point Pz is detected, the phase switching is performed after another 30° from the zero-crossing point Pz. Afterward, in the stage from 60° through 120°, the node Vu charges the node Vw through the neutral node Vc, and the node Vv is floating. The voltage Vv is compared with the voltage Vc, and when the zero-crossing point Pz is detected, the phase switching is performed after another 30° from the zero-crossing point Pz. An electrical cycle from 0° through 360° is completed by repeating the aforementioned steps for different nodes. Such phase switching mechanism by detecting the zero-cross point between a floating phase and the BEMF is applicable even when the rotation speed is controlled by a pulse width modulation (PWM) method, or when a soft-switching method is used to trim a phase current into a quasi-sinusoidal waveform, except that the range for detecting the zero-crossing point Pz is different.
The rotation of the motor is generated by a torque and the torque is generated by an interaction between the electromagnetic field of the stator and the permanent magnetic field of the rotor. For a motor to operate with low noise, the magnetizing field on the permanent magnet needs to be a sinusoidal waveform, and the induced BEMF in each phase coil also needs to be a sinusoidal waveform; furthermore, the phase current also needs to be a sinusoidal waveform and should be in phase with the BEMF, so that the motor can operate by an optimal efficiency. If the floating phases are used to sense the zero-crossing points with the BEMF, every phase needs to be floating in turn and its current in the floating period is zero; hence, the phase current can not be a sinusoidal current waveform, and the torque generated by the interaction between the electromagnetic field generated from the stator and the permanent magnetic field of the rotor will present a transient imbalance, resulting in vibrations and noises during phase switching. Although there are techniques such as quasi-sinusoidal waveform signal processing that can reduce the phase switching noises by narrowing the floating time period, there are still considerable noises which can be troublesome in a quiet environment or in a place demanding high quietness such as in a library.
U.S. Pat. Nos. 8,093,847, 7,034,478, and 5,491,393 disclose different solutions for lowering noises during motor phase switching; however, the noises due to floating phases are still unsolved.